A flame sensor is a component on the furnace that is responsible for igniting and maintaining the flame within the furnace. A flame sensor failure is relatively common due to buildup of materials and particles in the confined area of the furnace. Therefore, it is important to regularly remove and clean the flame sensor in order to keep the furnace running correctly.
Most importantly, turn the furnace off. It can be turned off using the switch on the unit or by cutting the power source at the breaker box.
Locate the flame sensor. It is the metal rod located in the direct path of the burner with a wire attached to it. Often, it has a white ceramic base.
Remove the sensor (if possible) by removing the screws and pulling the wire off. In addition, unscrew the bracket holding the flame sensor in place and remove from the furnace.
Clean the surface of the sensor by wiping with sandpaper (approximately 400 grain) or with steel wool. If you do not have either available, it can also be gently scraped with a knife to remove the build up and debris.
Once clean, reinstall the flame sensor directly back into the location from which it was removed. Once secured, turn power to furnace back on and test furnace to ensure it turns on properly.